ListSelectionListener getting cell value and assigning it to a variable

[Liam McTavish]

i have set up a selectionListener on a JTable to select an id of a particular row. ID is a column in the row (col 0). i checked this within the ListCustPanel class that follows by having it printout the selectedID, it worked. but ive added the method getSelectedID() to get the selectedID for other classes that use the Panel, and it seems that the variable never gets assigned. whats the problem here?

class ListCustPanel extends JPanel implements ListSelectionListener{

	private static final long serialVersionUID = 1L;
	private Object selectedID;
	private JTable table;
	
	public ListCustPanel(travelagent agent){
		
		String[] colNames = {"ID", "First Name", "Last Name"};
		
		int size = agent.getCust().size(); //get the size of the arraylist in order to define an object array to contain information in
		Object[][] data = new Object[size][3]; // create the array
		
		int i = 0;
		Iterator<customer> iter = agent.getCust().iterator(); //create an iterator to progress thought the arraylist
		while(iter.hasNext()){
			customer temp = iter.next(); //assign the current customer to a variable
			
			//extract its attributes and feed into object array
			data[i][0] = temp.getID(); 
			data[i][1] = temp.getFname();
			data[i++][2] = temp.getLname();
 			
		}
				
		// convert the object array into a JTable
		DefaultTableModel custList = new DefaultTableModel(data, colNames);
		table = new JTable(custList);
		table.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
		
		JTableHeader header = table.getTableHeader();
		header.setBackground(Color.GREEN);
		// create a sorter to sort table rows
		TableRowSorter<TableModel> sorter = new TableRowSorter<TableModel>(table.getModel());
		table.setRowSorter(sorter);
		ArrayList<SortKey> sortKeys = new ArrayList<RowSorter.SortKey>();
		sortKeys.add(new RowSorter.SortKey(2, SortOrder.ASCENDING));
		sorter.setSortKeys(sortKeys);
		
		ListSelectionModel lsm = table.getSelectionModel();
		lsm.addListSelectionListener(this);
		
		JScrollPane scrollPane = new JScrollPane(table);
		table.setFillsViewportHeight(true);
		add(scrollPane);
			
	}

	
	public void valueChanged(ListSelectionEvent e) {
		if(!e.getValueIsAdjusting()){
			int i = table.getSelectedRow();
			selectedID = table.getValueAt(i, 0);
			
		}
		
	}
	
	public Object getSelectedID(){
		return selectedID;
	}
		
}

so what im trying to say is i have another JFrame class as such, and im getting a null pointer exception from where i use the getSelectedID() method:

class CustomerFrame extends JFrame implements ActionListener{
      private Object selectedID;

      public CustomerFrame(travelagent agent){
		setLayout(new GridLayout(2,1,5,5));
		
		setSize(800,500);
		setLocationRelativeTo(null);
		setTitle("Actions for customer");
		
		
		
		lp = new ListCustPanel(agent);
                add(lp);
                selectedID = lp.getSelectedID();
                System.out.println(selectedID.toString()); //produces a null pointer exception
     }
}

[Ranganathan Kaliyur Mannar]

Liam McTavish wrote:

                System.out.println(selectedID.toString()); //produces a null pointer exception

this means, no row has been selected by the user yet. This code is present in the constructor of the frame. How would the user select a row before the JFrame, JPanel are constructed and shown to the user?